1. Field of the Invention
This invention relates to variable magnification viewfinders of real image type and, more particularly, to such variable magnification viewfinders which are used with photographic systems in separation therefrom, as the systems are of external finder type. Still more particularly, this invention relates to an objective lens system constituting part of the variable magnification viewfinder, which is designed by setting forth appropriate rules to improve the viewfinder image at all magnifications, so suited to be used in, for example, still cameras or video cameras.
2. Description of the Related Art
In the camera having a viewfinder system and a photographic system formed separately from each other, it is conventional that, if the photographic system is a variable magnification system, the viewfinder system, too, is configured as a variable magnification system, and is arranged so that the field magnification of the viewfinder system is made to vary according to the variation of magnification of the photographic system. For the variable magnification viewfinder, because of having to be set in the camera, it is required in general that the bulk and size be minimized and, moreover, that the configuration is amenable to the easy technique of obtaining as much zoom ratio as desired.
So-called 2-unit type variable magnification viewfinders have been variously proposed in which an objective lens system is constructed with two lens units of negative and positive refractive powers, respectively. Both of the two lens units axially move to vary the magnification. An object image formed by the objective lens system at each of various magnifications is converted by a Porro-prism into a non-inverted erecting image, which is then observed through an eyepiece lens system.
Also, Japanese Laid-Open Patent Application No. Hei 4-51108 discloses a variable magnification viewfinder whose objective lens system is constructed with three lens units of minus-minus-plus refractive power arrangement in this order from the object side. The magnification is varied by moving the second lens unit toward the object side and the third lens unit toward the image side.
Another variable magnification viewfinder is disclosed in Japanese Laid-Open Patent Application No. Sho 64-65519, in which an objective lens system is constructed with three lens units, of which the first is negative in refractive power, the second is positive and the third is positive. In this case, the first and second lens units are selected to be movable for varying the magnification.
Also, Japanese Laid-Open Patent Applications Nos. Sho 61-156018 and Hei 1-116616 have proposed a real image type of variable magnification viewfinder whose objective lens system is constructed with a multiplicity of lens units. The air separations between any adjacent two of the lens units are made variable for varying the magnification. An object image formed by the objective lens system at each of various magnifications is converted by an image inverting member such as a Porro prism into an erecting image, which is then observed through an eyepiece lens system.
Of the variable magnification viewfinders described above, the 2-unit variable magnification viewfinder, that is, whose objective lens system comprises the negative first and positive second lens units, both lens units moving axially to vary the magnification, has a feature that the second lens unit contributes to variation of the magnification, while the first lens unit compensates for the shift of the image plane. Therefore, the first lens unit has to be used in correcting the eyesight change. Another problem is that good stability of aberration correction against variation of the magnification is difficult to maintain from the point of view of the lens configuration.
In the case of the 2-unit variable magnification viewfinder, if the manufacturing tolerance is to be absorbed by adjusting the eyesight or by adjusting the parallax, some lens units must change their positions in a different way from that when zooming. This leads to another problem that the complexity of structure of the operating mechanism increases greatly. Besides this, if zooming from one end of the range to the other end accompanies reciprocating of a lens unit, the viewfinder image is often caused to sway at the turning point of movement of the lens unit, thereby giving an additional problem.
In general, the variable magnification viewfinders of real image type have a feature that, in order to attain an increase of the zoom ratio to a predetermined value with the limitation of the bulk and size to a minimum, there is need to set forth proper rules of lens design, in particular, for the objective lens system. Taking an example of the variable magnification viewfinder proposed in the above-described Japanese Laid-Open Patent Application No. Sho 64-65519, if the zoom ratio is increased, the amount of movement of the second lens unit would also be increased. Thus, the size of the entire lens system is caused to increase. To assure minimization of the size of the entire lens system, the refractive powers of the first and second lens units may be strengthened. If they are made merely stronger, the aberrations vary to a greater extent with variation of the magnification. Therefore, it becomes difficult to observe the viewfinder image with good quality.